OK, so I can get the screw undone for the main waste at the bottom (cracked it off, did it back up tight, filled bath, checked for leaks) but I can't change the waste properly until I get this bleedin overflow off. I've managed to crack the silicone off it with some stilsons (so now it's gotta come out!), but can't figure out how the inside and outside parts are held together? Does it unscrew, just pull apart or what - don't mind damaging it a bit to get it off but don't want to wreck the bath removing it!